Impacts of increasing wildfire severity on long-term carbon dynamics of Alaskan boreal forests

Background/Question/Methods: Climate-sensitive disturbances, such as wildfire, can feed back positively to climate warming via the carbon (C) cycle if C released by combustion is not replaced over post-fire succession. In boreal forests, burning of old carbon in deep organic soils is not only an important determinate of ecosystem element balance over the disturbance cycle, but also sets the conditions that control tree seedling recruitment, species dominance and successional dynamics. Species dominance, in turn, has the potential to exert strong control over the plant-soil-microbial feedbacks that determine C and nutrient coupling, C storage, and ultimately, replacement of combusted C. We examined the consequences of increasing fire severity for long-term C balance and C and nitrogen (N) coupling in Interior Alaska boreal forests that comprise the Regional Site Network of the Bonanza Creek LTER. We estimated C and N combustion losses in 80 stands of the historically dominant conifer species, black spruce (Picea mariana), that burned in 2004. Over the next decade, we followed tree regeneration in these stands and used seedling species dominance to identify the emergence of post-fire successional trajectories. In addition, we assembled data from 270 stands that varied in time after fire and successional trajectory, and estimated C and N dynamics across 150 years of post-fire succession for dominant trajectories.

Results/Conclusions: Our results show that fire-induced shifts in dominant plant species can introduce novel pathways for resilience of C cycling to disturbance. In these Alaskan forests, deep burning shifted tree dominance from slow growing, historically dominant spruce to fast growing deciduous tree species (Populus tremuloides and Betula neoalaskana). Ensuing plant-soil-microbial feedbacks catalyzed transfer of the limiting nutrient N from low C:N surface organic soil to high C:N aboveground plant biomass, resulting in a five-fold net increase in C storage over the disturbance cycle. Stored C in deciduous trees could persist longer on the landscape than in spruce ecosystems if flammability feedbacks increase the fire-free interval and reduce future fire severity, and life history traits reinforce self-replacement. Whether this shift in species composition acts as a negative, mitigating feedback to climate warming will be determined by the largely unknown fate of these transitional deciduous forests.